Spring Reverb tank - how to match impedance

[Yazoo]

I am going to build the spring reverb project at Westhost: http://sound.westhost.com/project34.htm

It's Figure 4 on the page. I've got an Accutronics 8DB2C1B tank and the specs say it is 310 Ohm/36 Ohm (Input impedance/DC) 2575 Ohm/200 Ohm (Output Impedance/DC).

The Westhost schematic says that resistors R3L, R4L and capacitor C2L should be selected to match the tank being used. What would be the best values to use? I have to be honest, I've got no idea how to work this out myself.

[jonnyeye]

From the article, just above figure 4:

"For a great deal more information about how to drive reverb tanks properly, including measurements and tables with optimum values, see Care and Feeding of Spring Reverb Tanks."

That page (specifically Table 2 and the text just beneath it) tell you how to choose values for those three parts.

[Yazoo]

Thanks, I hadn't looked at that. I'm guessing the values for the 250 Ohm coil impedance should be close enough.

[merlinb]

It's not at all critical. Some people drive their tanks from a low-impedance voltage source, others from a high-impedance current source (as in ESP's example). All approaches work; spring reverbs are pretty stone-age devices, and there are no fixed rules for them.

[PRR]

> 8DB2C1B  ... 310 Ohm/36 Ohm (Input impedance/DC)

Then per Table 2 of Care and Feeding page, you only need 7mA peak, which is WELL inside the ability of a naked NE5532. So you don't need the discrete transistor buffer. (That's only needed for A impedance, 8 Ohms.)



As Merlin says, "matching" is not the goal; the impedance is NOT "310 Ohms" except at one frequency. There's some method in using hi-Z drive (jiggle is proportional to current), which is why Rod returns the tank through R3 (to sense coil current). Lo-Z works too, though may tend to be dark. But spring reverb bandwidth is so small that bright/dark is not a big deal.

The R3 R4 ratio sets gain from source to tank, and may need changing for 8 or 1,475 Ohm coils; but 150-600 is, as you say, "close enough".

[amptramp]

You only need to match impedance if you are looking for maximum power transfer.  This would be the case for a speaker, but a low-power item like a reverb tank, a lower drive impedance and a higher impedance at the output would be perfectly OK.